In rotary machines such as turbines, seals are provided between rotating and stationary components. For example, in steam turbines it is customary to provide a plurality of arcuate packing ring segments (sometimes referred to as seal ring segments) to form a labyrinth seal between the stationary and rotating components. Typically, the arcuate packing ring segments are disposed in an annular groove in the stationary component concentric to the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth-type seals, the seal faces carry a radially directed array of axially spaced teeth that are radially spaced from an array of axially spaced annular grooves forming the sealing surface of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. The sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.
In a typical installation, the annular groove is dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component (e.g., a housing or casing) is typically split lengthwise along a generally horizontal extending midline defining upper and lower halves of the stationary housing. Thus, the semi-annular dovetail grooves receive portions of the arcuate packing ring segments. The packing ring segments are similarly dovetail-shaped, having a pair of flanges directed axially away from one another for disposition within the dovetail groove and a neck which joins the seal face and the flanges of the segment and passes through the slot defined by the locating flanges of the groove. The neck carries the arcuate seal face radially inwardly of the groove when each segment is installed.
In positive pressure, variable clearance type packing rings, the segments are typically spring biased into outer or large clearance positions with the seal faces carried thereby spaced substantially outwardly of the rotary component. Thus, for example, at startup of the machine, the springs displace the segments radially outwardly. After start-up, the working fluid medium, e.g., steam, is inlet to the grooves of the stationary component, biasing the segments to move inwardly against the bias of the springs toward the inner or small clearance positions. It will be appreciated that when the segments are retracted to their large clearance positions during startup and turbine shutdown by the springs, the gaps between the end faces of adjoining segments become increasingly greater. Conversely, when the segments are displaced inwardly, the gaps between the end faces of the segments decrease substantially to a fully-closed position when the segments obtain their smallest diameter. To accommodate the inward and outward movement of the segments, the segments are disposed within the grooves of the stationary component in a sliding or piston-fit arrangement.
In an effort to avoid possible hang-up or binding of the seal segments due to uneven friction forces and potentially open up large leakage gaps in comparison with the more conventional fixed or spring backed positive pressure packing ring segments, another form of a variable clearance packing ring seal between stationary and rotating components has been proposed which uses thermal expansion characteristics of the various elements to enable a large seal clearance between the packing ring segment sealing face and the rotating component at start-up, while ensuring a small clearance and minimal inter-segment leakage therebetween at steady-state operation (See U.S. Pat. No. 6,065,754, the entire disclosure of which is incorporated herein by this reference). According to that patent, there is provided a packing ring segment having a lower coefficient of thermal expansion than a seal holder, i.e., the stationary component, typically a turbine housing or packing casing. Also provided is a centering ring having a greater coefficient of thermal expansion than the seal holder. A pair of centering rings are provided on axially opposite sides of the neck of the packing ring segments and are supported by the stationary component, for example, by reroundable dowels affixed to the seal holder and projecting in an axial direction into the dovetail cavity. The packing ring segment is supported by the centering rings and each segment is biased radially inwardly by springs acting between the segments and the stationary component. The packing ring segments engage contact surfaces on the opposite ends of the centering rings to maintain a large clearance position at start-up between the seal faces of the segments and the rotary component. In that position, the segment ends are closed.
The centering rings have a greater coefficient of thermal expansion than the seal holder, rotor and packing ring segments. After start-up and as the temperature rises, the centering rings thermally expand in a circumferential direction to a greater extent than the packing ring segment. The contact surfaces, forming the support points for the packing ring segment on the center rings, are therefore shifted circumferentially to enable the packing ring segment to be displaced radially inwardly. As the machine reaches steady-state operation, the packing ring segments engage the outer surface of uniform diameter of the center rings, thus assuring concentricity of the packing ring segment sealing uniform clearance between the sealing faces of the segments and the rotary component and opening end gaps between the seal segments. The result is a uniform clearance seal that retains its labyrinth tooth geometry during start-up and a steady-state operation and has minimal bias leakage.
When the rotary machine (e.g., steam turbine) is at rest, for example, prior to startup, the segments, lying in their largest radially outward position, have been observed to slide along the circumferential groove of the stationary component in a downward direction under the action of gravity forces. As a consequence, an accumulation of clearances between the end faces of the segments appears in the upper housing, while the lower segments become closer, i.e., butting end-to-end. For example, when six segments are employed in a rotary machine, three segments are typically disposed in the upper housing of the stationary casing and three segments are disposed in the lower housing, i.e., respectively above and below the horizontal midline or splitline of the machine casing. Prior to startup, the side segments in the upper housing and the side segments of the lower housing may slide under the action of gravity forces such that the side segments of the lower housing engage the lowermost central segment in the lower housing of the machine, with the accumulated clearance between the end faces appearing between the uppermost segment in the upper housing and one or both of the side segments of the upper housing.
Upon startup, the segments are designed to move from their radially outward positions to their radially inward positions. Because of the effect of gravity, the segments tend to move in sequence. First, the top segment moves radially inwardly, followed by the side segments of the upper housing. The side segments of the lower housing then move radially inwardly, followed by the lowermost segment. It has been found, however, that because the segments become displaced circumferentially about the groove of the stationary component by gravity forces, the inward movement of the side segments of the lower housing of the stationary component, essentially in an upward and inward radial direction, can preclude or block radial inward (upward) movement of the lowermost segment. That is, the fluid pressure forces tending to displace the lowermost segment radially inwardly are insufficient to displace the lower side segments in a generally upward circumferential direction to enable the lowermost segment to move into its radially innermost position. Consequently, the lowermost segment remains radially displaced from its designed radially inward position resulting in a larger than desired clearance between its sealing face and the rotary component.
Thus, variable clearance positive pressure packing rings require both upper and lower support methods. These support methods have four design criteria that are generally considered to be critical to quality (the xe2x80x9cCTQxe2x80x9d): A) retain the upper packing rings during installation; B) prevent rotation of the packings due to windage or rubs; C) control the butt-clearance at the horizontal joints for both the upper and lower half sections; and D) prevent pinching out of the lower packing ring section in the lower half.
A variety of methods have heretofore been used for to retain upper packing rings, including: (A) a support bar which is bolted to the housing (See U.S. Pat. No. 5,709,388); (B) an axial dowel-type pin that intersects both the housing and the packing ring located in the upper half; and (C) an axial rivet, which is similar to the type B method. Lower segment retention has been more problematic.
Attempts have been made to accommodate the gravity forces acting on seal segments. See, for example, U.S. Pat. Nos. 5,464,226 and 5,395,124, the disclosures of which are incorporated herein by this reference. In the latter patent, so-called gravity springs are disposed in the stationary component to apply an upward biasing force to the seal segments in the lower housing. These gravity springs engage the seal segments intermediate their ends and at their centers of gravity. The seal segments, moreover, are provided with circumferential springs between the adjoining end faces, as well as between the end faces adjacent the horizontal midline. Anti-rotation keys are fixed to the upper and lower housing at the casing midline, the circumferential springs bearing against the anti-rotation keys. Consequently, the seal segments are biased outwardly by the springs between the segment ends and inwardly by fluid pressure forces. When the seal segments move inwardly, the gravity springs function such that the segments have little or no weight, permitting closure of the lower seal segments to the inner position. Thus, the lower seal segments are said to float by virtue of their opposing circumferential and gravity spring forces and such segments move radially inwardly and outwardly. Also, the end circumferential springs bearing against the anti-rotation keys bias the lower seal segments for circumferential displacement within the grooves in contrast to the present invention wherein circumferential displacement of the lower side segments is positively prevented and horizontal, rather than radial, movement of the lower side segments between inner and outer positions is provided.
According to another development, as disclosed in commonly owned U.S. Pat. No. 5,709,388, the disclosure of which is incorporated herein by this reference, a guide is provided for preventing circumferential slippage of the segments about the groove of the stationary components. More specifically, according to the invention of the ""388 patent, each of the side segments in the lower housing of the casing is provided with a guide which supports the segment, prevents the segment from circumferential displacement under gravity forces, and enables the segment to slide horizontally between outer large and inner small clearance positions, respectively. In a preferred form, each guide comprises an angle bracket, one leg of which is secured to the outer face of the segment adjacent the end of the segment near the midline of the rotary machine. The generally horizontally projecting other leg of the bracket is received in a recess formed in the interior surface of the stationary component and rests on a stop carried by the stationary component. The stops prevent downward circumferential displacement of the side segments in the groove of the lower housing of the rotary machine and thereby maintain clearance between the butt ends of adjacent packing segments. The bracket also supports the side segments of the lower housing of the machine for movement in a generally horizontal direction toward and away from a vertical plane through the axis of the machine.
To provide added performance to the unit, standard packing rings are desirably replaced with variable clearance positive pressure packing (VCP) of the type described hereinabove. With the replacement of standard packing rings with VCP, a lower ring segment support method must utilized for CTQ xe2x80x9cDxe2x80x9d. However, standard packing ring and housing design utilizing the doweled pin upper section retention method or the other methods mentioned above can not have lower variable clearance positive pressure packing (VCP) applied without substantial housing modifications. For example, these modifications would include substantial machining of support bar pockets in the lower half housing part to create a method for supporting a lower half VCP sections, as described in U.S. Pat. No. 5,709,388. The housing modifications increase power plant outage cycle time by forcing the lower half housing parts to be removed and most often machined off-site, thus increasing the cost of the conversion process.
To apply VCP to housings currently utilizing standard retractable packing rings with minimal lower half housing modifications, the present invention proposes to utilize the existing housing horizontal joint seal key assemblies for lower half VCP section retainment, thus reducing the power plant outage cycle time and reducing outage costs. The retention assembly thus provided also reduces the number of parts required for retention and provides a more simple design than the prior art.
Thus, the invention is embodied in an apparatus for preventing circumferential displacement of a lower half packing ring side segment mounted to a stationary component of a rotary machine having a component rotatable about a rotary axis, the stationary component having an annular groove about the axis, at least one axially directed locating flange about the axis and in part defining a slot opening into the annular groove, the stationary component being defined by upper and lower half housing parts secured to define a horizontal midline joint, the apparatus comprising: a lower side segment body for forming a lower side segment one of a plurality of annular segments about the axis of the machine and for partial disposition in the annular groove of the stationary component; the lower side segment body having upper and lower circumferential ends, an arcuate seal face, at least one axially directed flange for disposition in the annular groove, a neck portion receivable in the slot and interconnecting the flange and the seal face, and a length so as to extend circumferentially beyond, so that the upper circumferential end is disposed vertically above, the horizontal midline joint of the stationary component; and a key bar member secured to the stationary component and projecting into the annular groove for engagement with a key bar receptacle defined in a radially outer surface of the lower side segment body, adjacent the upper circumferential end, for preventing downward circumferential displacement of the lower side segment body in the annular groove.
In an exemplary embodiment a part of the key bar member is disposed in a key bar groove defined in the stationary component.
The invention is also embodied in a rotary machine comprising: a component rotatable about an axis; a stationary component including an annular groove about the axis, at least one axially directed locating flange about the axis and in part defining a slot opening into the annular groove, the stationary component being defined by upper and lower half housing parts secured to define a horizontal midline joint; a plurality of generally annular packing ring segments disposed about the axis, each segment having an arcuate seal face, at least one axially directed flange disposed in the annular groove and a neck portion in the slot and interconnecting the flange and the seal face; certain of the plurality of packing ring segments being movable between radial outward and radial inward positions relative to the axis, including a lower side segment defining a lower side one of a plurality of annular segments about the axis of the machine; the lower side segment having upper and lower circumferential ends and a length so as to extend circumferentially beyond, so that the upper circumferential end is disposed vertically above, the horizontal midline joint of the stationary component; and a key bar member secured to the stationary component and projecting generally radially inwardly therefrom for engagement with a key bar receptacle defined in a radially outer surface of the lower side segment body, adjacent the upper circumferential end, for preventing downward circumferential displacement of the lower side segment body in the annular groove.
The invention also relates to and may thus be embodied in a method of mounting a variable clearance positive pressure packing to a stationary component of a rotary machine having a component rotatable about a rotary axis, the stationary component having an annular groove about the axis, at least one axially directed locating flange about the axis and in part defining a slot opening into the annular groove, the stationary component being defined by upper and lower half housing parts secured to define a horizontal midline joint, the method comprising the steps of: providing a lower side segment body for forming a lower side segment one of a plurality of annular segments about the axis of the machine, the lower side segment body having upper and lower circumferential ends, an arcuate seal face, at least one axially directed flange for disposition in the annular groove, a neck portion receivable in the slot and interconnecting the flange and the seal face; partially disposing the lower side segment body in the annular groove of the stationary component by slidably engaging the at least one axially directed flange with the annular groove, the lower side segment body having a length so as to extend circumferentially beyond, so that the upper circumferential end is disposed vertically above, the horizontal midline joint of the stationary component; engaging a key bar member with a key bar receptacle defined in a radially outer surface of the lower side segment body, adjacent the upper circumferential end; and securing the key bar member to the stationary component to prevent downward circumferential displacement of the lower side segment body in the annular groove.